Even today, waste products of relatively easily degradeable plastics, such as polyurethanes, polyesters, polycarbonates and polyamides are still burned or dumped. Economically and ecologically, however, it would be much more desirable to degrade these hydrolytically degradeable polymers into their starting components which could then be reused for the production of plastics.
It is known that, for example, polyurethane foam waste can be degraded into its low molecular weight starting compounds at elevated temperature and pressure (for example 40 bars and 240.degree. C) in a stirrer-equipped autoclave, the polyisocyanate originally used in the synthesis of the polyurethane foam being hydrolyzed into the corresponding polyamine. The complete hydrolytic degradation of polyurethane foam waste under the conditions specified in such an autoclave would take about one hour. Any process of this kind, however, is only of economic and commercial interest if it can be carried out continuously.
Processes for hydrolyzing polyurethane foam waste are known and are described, for example, in German Offenlengungsschrifts Nos. 2,362,919; 2,362,920 and 2,362,921. According to the last of these Offenlegungsschrifts, hydrolysis is carried out in batches in a closed reaction zone whereas according to Offenlegungsschrifts Nos. 2,362,919 and 2,362,920 hydrolysis is carried out continuously (a) in a fluidized bed and (b) in a defined, tubular reaction zone, respectively. Unfortunately, these process are attended by a number of disadvantages.
Thus, the entire fluidizing gas must be heated to the reaction temperature (250.degree.-400.degree. C) in the reactor and subsequently cooled again in order to condense the diamine. Vast quantities of energy are necessarily wasted. Additionally, the reaction zone used to carry out hydrolysis has to be extremely large because the already extremely voluminous foam takes up even more space as a result of fluidization. Additionally, hot, unhydrolyzed and partially hydrolyzed foam particles have a marked tendency to stick together. The reaction zone which is not continuously scraped by suitable means very soon becomes blocked up. Finally, the individual foam particles are surrounded by a coating of hydrolysis product. Unfortunately little or no reaction takes place inside them since no shear forces act upon the particles.